1. Field of the Invention
The present invention relates to a holographic recording apparatus.
2. Description of the Related Art
In order to achieve high-density information recording, hologram draws attention, which allows a data signal to be recorded with high density. A feature of this hologram is to record wavefront of light carrying information to be recorded onto a recording medium formed of a photosensitive material, such as a photorefractive material, as change of refractive index in terms of volume. For example, a recording and reproduction system that uses a holographic recording medium in the form of a disk (holographic disk) has been developed (see Japanese Patent Kokai No. 11-311937).
FIG. 1 shows the principle of holographic recording and reproduction. A holographic disk 1 as an exemplary holographic recording medium used in this holographic recording and reproduction includes a hologram layer, a reflection film, and a protection layer that are formed over an entire principal surface of a transparent substrate in the form of a disc in that order. In a reflection surface of the reflection film of the holographic disk, servo areas 2 are provided at intervals of a predetermined rotation angle and a data area 3 is provided in a fan-like section between the adjacent servo areas 2.
The holographic recording and reproduction requires an objective lens for making signal light and reference light incident on such a holographic disk. The objective lens 4 is positioned by an actuator 5 that can move in a thickness direction and a track direction of the holographic disk. In the case in which laser light emitted from the objective lens does not follow a track on the holographic disk because of eccentricity of the holographic disk or the like, the actuator 5 moves the objective lens 4 based on a signal from a servo controller (not shown), thereby that laser light can be incident on the data area 3 correctly.
In the holographic recording and reproduction, reference light is made incident on the holographic disk 1 by using the objective lens 4 so as to pass through the hologram layer and be converged as a spot on the reflection film. The reference light reflected by the reflection film passes through the hologram layer again. Moreover, signal light carrying information to be recorded is also made to pass through the hologram layer by using the objective lens 4. Thus, the reflected reference light and the signal light interfere with each other in the hologram layer to generate an interference pattern, thereby volume hologram is recorded in the hologram layer. In addition, recorded information can be reproduced by irradiating the holographic disk with the reference light and detecting and decoding reproduction light that is restructured from respective holograms.
In an apparatus for performing the aforementioned holographic recording and reproduction, the center of the aperture area of the objective lens (i.e. effective diameter of the lens) may be off from the optical axis of the signal light. In this case, a part of the signal light may be prevented from passing through the aperture area of the objective lens and reaching the data area of the holographic disk. In other words, a part of information to be recorded and being carried by the signal light may not be recorded.
Moreover, in order to record information in the hologram layer of the holographic disk by a holographic recording and reproduction apparatus, the data area has to be irradiated with the signal light for a predetermined time, for example, because a response time of the photosensitive material used as the hologram layer is long. Thus, in the case in which the holographic disk is to be rotated, it is necessary to perform positioning of the objective lens in such a manner that the objective lens follows the data area in order to irradiate the data area with the signal light for the predetermined time. However, during the positioning operation, the center of the aperture area of the objective lens may deviate from the optical axis of the signal light.
Therefore, an exemplary problem to be solved by the present invention is to provide a holographic recording apparatus that can make signal light carrying data to be recorded incident on a recording medium so as to correctly record that data onto the recording medium, even if an optical axis of an objective lens is displaced with respect to an optical axis of the signal light.